Radiative forcing particles

Sulfur 10-200% increase6 in surface area of sulfate particles Increased aerosol mass loading, change in radiative forcing... 

In the Charlson et al. (1992a) and Penner et al. (1994) estimates, Bsoi = 3.3 X 105 mol m-2, with uncertainties in QSOi, ysullate, and rsullatc of 1.15,1.5, and 1.5, respectively. (Note, however, that some subsequent studies have suggested that direct radiative forcing may vary nonlinearly with the sulfate concentration due to chemical interactions with other particle constituents e.g., see West et al., 1998.)... 

FIGURE 14-29 Calculated direct radiative forcing due to sulfate aerosol particles (adapted from Penner et al., 1998). 

It should be noted that the magnitude of the predicted forcing is quite sensitive to treatment of relative humidity (RH) in the model because of the effects on particle size and optical properties (e.g., Haywood and Shine, 1995 Haywood and Ramaswamy, 1998 Ghan and Easter, 1998 Haywood et al., 1998a Penner et al., 1998). For example, in the calculations by Penner et al. (1998), when the particle properties were held fixed at the values for 90% RH for 90-99% RH, the predicted direct radiative forcing for sulfate particles decreased from - 1.18 W m-2 to -0.88 W m 2 for the Northern Hemisphere and from -0.81 to -0.55 W m-2 globally. 

FIGURE 14.31 Calculated direct radiative forcing by sulfate, biomass, and fossil fuel black carbon (BC) + organic carbon (OC) particles in the (a) Northern Hemisphere, (b) Southern Hemisphere, and (c) global average (adapted from Penner et at., f998). 

In short, while net radiative forcing is a convenient means for examining the potential importance of various anthropogenic perturbations for climate, it cannot be used in an additive manner for gases and aerosol particles to predict the ultimate impacts. 

The impact of secondary aerosols on indirect radiative forcing is the most variable and is the least understood [3]. The reasons why the indirect effect of secondary aerosols is so difficult to describe is that it depends upon [1] (1) a series of molecular-microphysical processes that connect aerosol nucleation to cloud condensation nuclei to cloud drops and then ultimately to cloud albedo and (2) complex cloud-scale dynamics on scales of 100-1000 km involve a consistent matching of multiple spatial and time scales and are extremely difficult to parameterize and incorporate in climate models. Nucleation changes aerosol particle concentrations that cause changes in cloud droplet concentrations, which in turn, alter cloud albedo. Thus, macro-scale cloud properties that influence indirect forcing result from both micro-scale and large-scale dynamics. To date, the micro-scale chemical physics has not received the appropriate attention. 

FIGURE 20.2 Illustration of the size (below particle) and number of molecules (upper right) in atmospheric particles ranging from critical clusters up to cloud drops. The visible spectrum is shown to emphasize the size range (0.1 to 1.0 mm) relevant to aerosol radiative forcing. Nucieation creates new particles that evolve into larger particles relevant to aerosol radiative... 

Reduced availability of SO2 would reduce the formation rate of new particles. Further studies are needed to elucidate the combined effect of these reactions of cloud microphysics and radiative forcing. Regions that cloud be affected most by these changes are the large regions of marine stratocumuli, especially the tradewind systems. 

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